This invention relates generally to a highly accurate index interferometric instrument, and more particularly, to one that utilizes a first light source with a bandwidth ranging from approximately 10 nm to 400 nm and a second light source with a bandwidth of less than 10 nm, such as a laser, to create interference patterns, which can be used to accurately calculate the index of refraction of an entire sample at any given point.
The index of refraction of a material and its homogeneity give important information as to the nature of the material, its impurities, its stoichiometry and the uniformity of the impurity distribution. A number of techniques have been used to measure the index of refraction and its variation throughout a material sample. One method is to dip samples into index-matching liquids. However, this method is not very accurate nor does it give a good index profile. Another method requires the construction of triangular prisms of the material, which wastes material and loses information because of index variation along the beam path.
A conventional interferometer can be used to measure the index of refraction of thin materials with poor accuracy, but is ineffective in measuring a material sample with a thickness in excess of 10 microns. In such an interferometer a single laser beam is divided, with one part of the beam being directed towards a reference mirror and the other part of the beam being directed towards a sample, the sample being mounted to a mounting mirror. When the light reflected back from the sample and the reference mirror are recombined three interference patterns can be created. They represent the reflection of the laser light from the front of the sample, the reflection of the light from the back of the sample, and the reflection of the light from the mounting mirror. In addition to these three interference patterns a number of interference fringes are also created, which are almost indistinguishable from the three interference patterns.
If the sample is very thin, one can use such an interferometer to determine the thickness of the sample and determine the optical pathlength of the sample, which together can be used to calculate the index of refraction. The thickness of the sample can be determined by counting the number of interference fringes or fraction of fringes between the interference pattern created by the reflection from the front surface of the sample and the pattern created by the reflection from the mounting mirror. The optical pathlength of the sample can be determined by counting the number of fringes between the pattern created by the reflection from the front surface of the sample and the pattern created by the back surface of the sample. The index of refraction can be calculated by utilizing the formula n=(p/t)+1, where n=the index of refraction, p=the optical pathlength of the sample, and t=the thickness of the sample. However, for a sample with a thickness of much greater than one fringe, it is very difficult to locate interference patterns and differentiate these patterns from interference fringes.
If the interferometer utilizes a white light source instead of a laser, localized, and easily distinguishable interference patterns can be created. However, if the sample is relatively thick, the white light source will be dispersed as it passes through the sample and will not be able to create an interference pattern from the back of the sample. Also, the white light source does not have interference fringes continuously from the front surface reflection to the back surface reflection. Thus, the distance between each of the patterns can not be measured for a thick sample utilizing white light.
It would be highly desirable if an instrument were available which could be used to accurately measure the index of refraction of an entire sample.